1. Field of the Invention
The present invention relates generally to the power tools and electric motors for controlling such tools. More particularly, the invention relates to a microprocessor-based or microcomputer-based control circuit for monitoring tool operating conditions, such as thermal overload conditions, and for providing a unique warning or indication when a given overload condition has been reached.
2. Description of the Prior Art
In controlling the speed of an electric motor for use in power tools, it is now generally known to use gated electronic power controlling devices, such as SCR's or triacs, for periodically transferring electrical energy to the motor. Many popular power tools employ universal motors, which are readily controllable using such gated controlling devices.
Generally speaking, gated speed control circuits work by switching the motor current on and off at periodic intervals relative to the zero crossing of the a.c. current of voltage waveforms. These periodic intervals occur in synchronism with the a.c. waveform and are measured in terms of a conduction angle, measured as a number of degrees. The conduction angle determines the point within the a.c. waveform at which electrically energy is delivered to the motor. For example, a conduction angle of 180.degree. per half cycle corresponds to a condition of full conduction, in which the entire, uninterrupted alternating current is applied to the motor. Similarly, a 90.degree. conduction angle corresponds to developing the supply voltage across the motor commencing in the middle of a given half cycle, and thus corresponds to the delivery of approximately half of the available energy to the motor. Conduction angles below 90.degree. correspond to the transfer of even lesser quantities of energy to the motor.
With most power tools it is desirable to have some form of overload protection to warn the tool operator when excessive motor temperatures have been reached. In accordance with the teachings of U.S. Pat. No. 4,307,325, entitled "Digital Control System for Electric Motors in Power Tools and the Like", issued to Saar on Dec. 22, 1981, it is now known that the temperature of a power tool motor can be inferred from information already available to the motor speed control circuit. More specifically, the factors which control the temperature of the motor are the current drawn by the motor and the means provided for dissipating the heat generated by the motor. In most power tools a cooling fan is driven directly by the armature of the motor, and thus the cooling effect contributed by the fan can be determined from the measured speed of the motor. In addition, the current drawn by the motor can be determined from the speed of the motor and from the conduction angle at which the gated electronic power controlling devices are operating.
As more fully discussed in the Saar reference, many motor control circuits prior to Saar simply establish a maximum current level for determining an overload condition. A disadvantage with this approach is that it fails to recognize that it is not solely the instantaneous current draw of the motor which determines whether or not it will overload. Not only are the effects of cooling not accounted for in this approach, but also there is no recognition of the time factor involved. Cooling effects momentarily aside, a change in current does not immediately invoke a corresponding change in motor temperature. Rather, the temperature of the motor will, at any given point in time, depend on the amount of current being drawn and on the period over which such current has been drawn. Taking these factors into account, the overload protection scheme disclosed in the Saar patent utilizes, in effect, a numerical integrator in order to distinguish between safe operating conditions and the imminent occurrence of overload conditions.
Related to the problem of overload detection is the problem of how best to inform the tool operator when overload occurs. The warning must be clear, recognizable, and distinguishable from other warnings and operating modes. The traditional approach to providing warnings is through the use of indicator lights and audible alarms. Practical experience has shown that tool operators normally look at the cutting end of the implement and will therefore fail to notice flashing indicator lights located on the tool body. Audible alarms are likewise ineffective. The typical audible alarm, like the ones used in smoke detectors, produces sound pressure levels from 70 to 75 dB. A working drill, for example, creates sound pressure levels from 90 to 100 dB and will therefore mask the sound of the audible alarm. In addition, both lights and alarms also require additional power supply and interface circuitry which increase the cost of the tool and take up space, making the tool heavier and bulkier.